Because of their reliability and tolerance of high temperature processing, tungsten ohmic and Schottky contacts are highly desirable in III-V microelectronic devices. However, it is difficult to form patterns of tungsten with the precision required in modern microelectronic devices.
Tungsten can be readily deposited in thin layers at low temperature using rf sputtering. However efforts to pattern the deposited film encounter the difficulty that tungsten etches isotropically. While it is desired to etch only the tungsten unprotected by a photoresist mask, the tungsten also etches laterally, undercutting the mask. As a consequence, vertical profiles and critical feature dimensions, such as submicron FET gates, cannot be successfully transferred to the underlying tungsten using conventional photolithography.
Various attempts have been made to enhance the precision of pattern transfer to tungsten, but none provide a clear solution to the problem of forming tungsten contacts on III-V semiconductors. For example, Shattenburg et al. report using CBrF.sub.3 or CHF.sub.3 to etch tungsten while depositing polymer on the sidewalls to impede horizontal etching (J. Vac. Sci. Technol. B3, 272 (1985). The result is transferred features which are inflated at the base. Others report that using SF.sub.6 or CF.sub.4 plasma discharges create fluorine radicals which undercut masks and result in loss of feature size. See, for example, Suzuki et al., J. Electrochem. Soc. 129, 2764 (1982). Gas mixes of SF.sub.6 --CBrF.sub.3 or SF.sub.6 --CHF.sub.3 reduce undercut, but nonetheless yield isotropic profiles. (See Tennant et al., J. Vac. Sci. Technol. B7, 1836 (1989). Recently vertical etching of tungsten in SF.sub.6 at low temperatures has been reported, but the method requires unique etching equipment and is not well suited for commercial manufacture. See S. Tachi et al., J. Vac. Sci. Technol. A9,796 (1991).